build: refactor platform checks

also improve verbosity when downloading dependencies fails

backend.native/build.gradle

@@ -92,7 +92,7 @@ String[] linkerOpts1() {
                     '-lLLVMCore',
                     '-lLLVMSupport']
     List<String> ldflags = new ArrayList<>()
-    if (System.properties['os.name'].startsWith("Linux"))
+    if (isLinux())
         ldflags.addAll(["-L$llvmDir/lib",
                 "-Wl,-Bstatic", "-Wl,--whole-archive"]
                 + libs

backend.native/tests/build.gradle

@@ -83,7 +83,7 @@ abstract class KonanTest extends DefaultTask {
     }
 
     private linux() {
-        return System.properties['os.name'].startsWith('Linux')
+        return project.isLinux()
     }
 
     protected String linkDl() {